The U.S. Federal Reserve's portfolio of Treasury and mortgage-backed securities, assembled through three rounds of purchases starting during the financial crisis, has dropped below $4 trillion for the first time in more than four years, according to Fed data.

The value of Treasurys and MBS dropped to $3.997 trillion as of Wednesday, as the Fed nears rounding out the first year of a process of slowly cutting back its holdings. At its peak, the portfolio totaled more than $4.25 trillion, and including other assets was valued at more than $4.5 trillion.

The drawdown of assets on the Fed's balance sheet was set on "autopilot" a year ago so that holdings would diminish steadily without further action by policymakers.

Over the last decade, the Fed bought some $3.5 trillion in mortgage and Treasury bonds in an effort to boost riskier investments, hiring and economic recovery from recession.

In a nod to a stronger U.S. economy, the Fed since 2015 has raised interest rates well above zero, and since October of last year, begun shrinking its balance sheet to a more normal but yet-unspecified size. Next week the pace of roll-off from its portfolio will reach $50 billion per month and will continue at that rate until the central bank decides its holdings have reached an appropriate amount.

The Federal Reserve raised interest rates on Wednesday, as expected, and left its monetary policy outlook for the coming years largely unchanged amid steady economic growth and a strong job market.